The present invention relates generally to discharge plasma systems and methods, and more specifically to an apparatus and method for generating a glow discharge plasma under atmospheric pressure conditions.
Atmospheric Pressure Glow (APG) discharge is used in practise, among others, for non-destructive material surface modification. Glow discharge plasmas are relatively low power density plasmas, typically generated under atmospheric pressure conditions or partial vacuum environments.
In practise, the plasma is generated in a plasma chamber or plasma discharge space between two oppositely arranged parallel plate electrodes. The plasma is generated in a gas or a gas mixture in the discharge space by energizing the electrodes from AC power means.
It has been observed that a stable and uniform plasma can be created in, for example, a pure Helium or a pure Nitrogen gas. However, as soon as impurities or other gasses or chemical compositions at ppm level are present in the gas, the stability of the plasma will decrease significantly. Typical examples of stability destroying components are O2, NO, CO2, etc.
It has been observed that instabilities in the plasma will develop either in a high current density plasma or will extinguish locally the plasma. As a result, current arcing occurs and the glow discharge plasma can not be maintained. Instead, a combination of filamentary and glow discharge is generated.
Filamentary discharges between parallel plate electrodes in air under atmospheric pressure have been used to generate ozone in large quantities but are of limited use for the surface treatment of materials, since the plasma filaments tend to puncture or treat the service unevenly and are associated with relatively high plasma currents.
U.S. Pat. No. 5,414,324 discloses a plasma chamber having a first or upper electrode through which air is injected and a second or counter electrode on which the substrate to be treated is located. For protecting the substrate to be treated against sparks, i.e. current arcing in the plasma, an electrically conductive grid is provided between the oppositely spaced electrodes, which grid connects by an inductor with a large inductance (choke coil).
From experiments disclosed by U.S. Pat. No. 5,414,324 it appears that for practical use an inductor having an inductance of 100 H and even higher has to be applied.
Those skilled in the art will appreciate that the combination of conductive grid and high inductance is a design which can be used only for over-current protection, being in fact a solution for damping the current of the sparks developed in the plasma and preventing that the sparks will reach the counter electrode where the material to be treated in plasma is located. The solution disclosed in U.S. Pat. No. 5,414,324 can have only a small effect at normal low current operating conditions and can affect only slightly the plasma stability and will not protect against the formation of sparks. A spark is already there when the inductor starts to react.
Further, inductors of the size disclosed above require a relatively large amount of space and, due to the magnetic core required, have a significant weight. Further, the conductive grid limits the effective discharge space for treating a substrate by the plasma.
It is, therefore, an object of the present invention to provide a novel and improved apparatus and method for producing and sustaining a uniform glow discharge plasma.
This and other objects and features of the invention are achieved according to the present invention by an apparatus for producing and sustaining a glow discharge plasma in a plasma discharge space comprising at least two oppositely spaced electrodes, means for supplying a gas or a gas mixture under atmospheric pressure conditions in the discharge space, AC power supply means for energizing the electrodes, and electric stabilisation means for stabilizing current variations in the plasma, the stabilisation means connect between the electrodes and the power supply means, and are arranged for providing a negative feedback opposed to a positive feedback of the plasma to instabilities in the plasma.
Compared to the prior art solution as presented by U.S. Pat. No. 5,414,324 in the present invention, the plasma discharge space is not obstructed by the introduction of an electrically conductive screen and the like.
Electrically, an APG system can be represented by an equivalent RLC circuit diagram comprised of a capacitance C, mainly due to the capacitance of the spaced electrodes, an inductance L and a plasma resistance R. The inductance component includes APG self-inductance and the equivalent inductance of system coupling to the power supply. When the APG is coupled to a power supply by a transformer, the equivalent inductance of the coupling system is of a significant order of at least a few mH.
At plasma breakdown large current oscillations can be generated as a consequence of the current resonance generated by the RLC circuit. These current oscillations set a ground for the generation of waves in the plasma and the constriction of the plasma column. Due to the relatively low value of the APG self-inductance, the resonance frequency is high and is preferentially excited at breakdown. Accordingly, during the plasma breakdown high amplitude and current oscillations are developed which are a major risk for plasma stability. Also a strong electromagnetic emission is developed due to the self-resonance of the APG inductance and capacitance.
In a further embodiment of the invention, the stabilisation means are arranged for decreasing plasma voltage proportional to the rate of plasma current variation and/or to an electromagnetic field radiated by the plasma.
The present invention is based on the insight that the insertion of a negative feedback stabilisation means is functional at the plasma breakdown when the voltage applied to the plasma is decreased on a short time scale. To prevent the development of undesired high current oscillations, in a preferred embodiment of the invention the inductive component of the impedance of the stabilization means has to decreased substantially after the plasma breakdown. Also in order to prevent plasma instabilities initiated by low current streamers, the stabilization means are arranged in such a way that a negative feedback not only provided to the plasma current but also to the plasma electromagnetic radiation too. In this manner filamentary discharges in plasmas can be effectively avoided.
From experimentally obtained data, it has been shown that the solution according to the present invention results in a very stable plasma for a variety of different supply gases, excitation conditions, electrode designs and electrode spacings.
During breakdown, the plasma is in its most unstable phase in which even small voltage variations can easily affect plasma stability. By introducing a relatively large inductance, such as disclosed by U.S. Pat. No. 5,414,324, the voltage variations will be large and, as consequence, the plasma stability will be adversely affected.
In order to prevent the generation of the plasma oscillations and the reduction of the current amplitude after the critical phase of plasma breakdown, in the preferred embodiment of the present invention, the stabilization means are arranged for a substantial decrease of inductance after the plasma breakdown. Preferably, the stabilisation means are arranged for a substantial decrease of inductance at a plasma current density above approximately 1 mA/cm2. A decrease of inductance of at least one order of magnitude appears to be sufficient.
In a practical embodiment of the apparatus according to the present invention, the stabilisation means comprise inductor means. Those skilled in the art will appreciate that inductor means generate voltages opposed to the rate or speed of APG magnetic flux variation and APG current variation, dependent on their inductance.
By series connecting the inductor means and at least one of the electrodes to the power supply, a very simple electric circuit is obtained. For reasons of symmetry and obtaining a uniform as possible plasma, each of the electrodes of the APG apparatus series individually connects by inductor means to the power supply.
The inductor means, in a yet further embodiment, may comprise electronic inductor circuitry which, as such, are known to the skilled person. In a preferred embodiment of the invention, the inductor means comprise choke coil means.
Choke coil means having a core of ferromagnetic material comprising any of the group of ferrite which can saturate at low magnetic fields of order of 100 mT and which can operate at MHz frequencies, will provide a reduced impedance or inductance at plasma breakdown.
That is, by designing the core such that it saturates at plasma current values occurring during plasma breakdown, the impedance of the choke coil will reduce and large voltage variations will be avoided, and thereby additional plasma instability due to the choke coil. By directly series connecting the choke coil means with the power supply and the electrodes of the APG, the saturation effect will be directly effected by the plasma discharge current.
The use of inductors as such is a standard method to use in the design of the electronic circuitry as a simple way to filter high frequency voltage oscillations. However those skilled in the art will appreciate that the fast current variations associated with the plasma filamentation are rather current oscillations (plasma is the equivalent of a low impedance current source) and not voltage oscillations, and their damping is not straightforward. In most of the cases inserting a inductor in series with the stray capacitance of an atmospheric plasma reactor will form a LC resonant circuit and will generate large current oscillations which are increasing the plasma instability and the tendency to go in a filamentary form. Also a major seed for filamentation of atmospheric glow plasmas is the risk of generating high current density but of low current (of order of 1 mA) filamentary plasma types called streamers. Those skilled in the art will appreciate that due to low current, the development of streamer instabilities can not be straightforwardly prevented by the negative feedback to the streamer current generated by a inductor in series with the plasma.
In a practical embodiment of the invention, the inductor means can be designed for having an inductance of less then 10 H once the plasma has been produced. The inductor inductance before the saturation can be chosen using the formula given below:   L  ≈            10      5                      I        max            ⁢      τ      
wherein: L is the inductance of the choke coil;
Imax is the amplitude of current (without the use of inductor means);
xcfx84 is the breakdown development time (without the use of inductor means).
The required inductance is of the order of 10-50 mH/cm2.
In the case that the power supply means comprise an LC filter network (impedance matching network) used for the purpose of achieving optimum power transfer and filtering of the reflection of the plasma current to the power source, in accordance with the present invention, at least one of the electrodes connects by the inductor means to the LC filter network.
As was mentioned above in the case that the APG is powered through a transformer the coupling line has a considerable inductance which is not satisfying the design criteria required for an inductor stabilising means. In this case the coupling of the APG and of the inductor means to the transformer equivalent inductance must be weak in order to avoid the coupling of the APG to a unwanted inductance. To achieve this purpose the reflection of the plasma current in the secondary of the transformer must be reduced to minimum through the use of a filter network including LC, II or T networks.
In this case, the inductor means and the filter network have to be designed carefully for an effective damping of the high frequency components of the plasma current in the secondary of the transformer and to the power source current. A further advantage of the solution according to the present invention is that the inductor means, such as the choke coil means, provide a significant contribution to the reduction of reflected power to the power supply means.
Preferably, the inductor means should have a cut-off frequency above which their impedance is highly resistive, as a further measure of limitation of the current oscillations.
In order to have an effective as possible filtering of the undesired plasma electromagnetic radiation, the inductor means should be arranged in such a way that the flux of the plasma magnetic field through the inductor is maximized.
In an embodiment of this invention the inductor means are connected electrically as close as possible to one or both electrodes or to the plasma region.
In another embodiment of this invention the APG reactor itself is used as an inductor means by packing the APG in a ferromagnetic material.
In a preferred embodiment of the apparatus according to the invention, due to the fact that no screen for connecting the choke coil means and the like between the oppositely spaced electrodes is required, the electrodes can be spaced at a distance from 0.01 mm.
Due to the effective suppression of the current oscillations, the gas for generating the plasma can be selected from a group comprising Helium, Argon, Nitrogen, Air, Oxygen, Carbon Dioxide, and a mixture comprising any of the gasses of the group.
In a preferred embodiment of the invention, the gas is a mixture of noble gas such as helium or argon and a chemically active gas such as air, oxygen and carbon dioxide. The gasses may also comprise of specific chemical compositions which are usually involved in the chemical vapour deposition processes such as SiH4, hydrocarbons, organosilicons such as TEOS and HMDSO, and organo-metallics.
In a embodiment of this invention the slope of the AC voltage is arranged in such a way that the pulse duration is larger than the period of the resonant frequency of the inductor means and APG capacitance. In this way the stability of the plasma is substantially increased. Such a condition can be achieved if for example the AC frequency is higher than 50 kHz.
In a preferred embodiment of this invention the slope of the AC voltage is arranged in such a way that the time required for the development of plasma breakdown in all the volume of the APG is shorter than 200 ns. In this way the stability of the plasma is substantially increased. Such a condition can be achieved if for example the AC frequency is higher than 50 kHz.
The invention further provides a method for generating and sustaining a glow discharge plasma in a plasma discharge space comprising at least two oppositely spaced electrodes, by introducing a gas or gas mixture under atmospheric pressure conditions in the discharge space, by energizing the electrodes using AC power supply means and by stabilizing current variations in the plasma using electric stabilisation, the stabilisation means connect between the electrodes and the power supply means, and wherein the stabilization means operate to provide a negative feedback opposed to a positive feedback of the plasma to instabilities in the plasma.
In a preferred embodiment of the method according to the invention, the stabilisation means operate for decreasing plasma voltage proportional to the rate of plasma current variation and/or to the electromagnetic field radiated by the plasma.
In a preferred embodiment of the invention the stabilisation means operate to provide a substantial decrease of the inductive component of its impedance after plasma breakdown.
In a yet further embodiment of the method according to the invention, the stabilisation means operate to be substantially resistive above a specified frequency.
The apparatus and method according to the present invention can be used in practise for a wide variety of applications, such as but not limited to a device for plasma surface treatment of a substrate, such as surface activation processes which substrate can be glass, polymer, metal, etc., and generation of hydrophilic or hydrophobic surfaces; plasma device for the chemical vapour deposition process; plasma device for decomposition of gasses containing volatile ozone component; plasma device for removing toxical compounds from the gas phase; plasma device for surface cleaning purposes such as in the sterilisation or dry cleaning processes.
The above-mentioned and other features and advantages of the invention are illustrated in the following description with reference to the enclosed drawings.